In the production of seamless tube by the Mannesmann-mandrel mill process, first, a billet, which is a starting material, is heated to 1200 to 1260° C. in a heating furnace and after that, in the piercing-rolling process a hollow shell is produced by performing piercing-rolling using a piercer plug and the piercer rolls of a piercing-rolling mill. Next, a mandrel bar is inserted along the inner surface of the above-described hollow shell and elongation rolling is performed on a mandrel mill usually consisting of 5 to 8 stands by constraining the outer surface with grooved rolling rolls, whereby the thickness is reduced to a prescribed wall thickness and a material pipe or tube is produced. After that, the mandrel bar is extracted from the material pipe or tube and this material pipe or tube is sized on a sizing mill to a prescribed outside diameter to obtain a seamless pipe or tube as a product.
FIGS. 1A and 1B are diagrams showing an example of the schematic construction of a piercing-rolling mill. FIG. 1A is a side view and FIG. 1B is a plan view. FIG. 2 is a diagram showing an approximate positional relationship among the piercer roll, the piercer plug, and the billet. The illustration of the piercer plug is omitted in FIG. 1B, and for the sake of simplicity, the feed angle and toe angle of a pair of piercer rolls are set at 0 in FIG. 1B. As shown in FIGS. 1A and 1B, a piercing-rolling mill 10 is provided with a pair of piercer rolls 1a, 1b and a bullet-like piercer plug 3 whose rear end is supported by a mandrel 2. The pair of piercer rolls 1a, 1b are set in such a manner that the axial directions thereof are parallel to each other as viewed from the side or cross at a prescribed toe angle (in FIG. 1A, only the case where the piercer rolls are set parallel to each other is shown). On the other hand, the piercer rolls are disposed in such a manner that the two are inclined at a feed angle θ in directions reverse to each other as viewed from the plane and are configured to rotate in the same direction. The piercer plug 3 is disposed between the pair of piercer rolls 1a, 1b. 
In order to piercing-rolling a solid billet B using the piercing-rolling mill 10 having this configuration, first, the billet B is fed to between the pair of piercer rolls 1a and 1b. After the billet B is gripped by the pair of piercer rolls 1a, 1b the force with which the billet B is rotated by the frictional force of the piercer rolls 1a, 1b and the force with which the billet B is moved forward in the axial direction act simultaneously on the billet B. And until the billet B reaches the front end of the piercer plug 3, a compressive stress and a tensile stress act alternately continuously on the central part of the billet B (the rotary forging effect) and an opening becomes tend to be formed. When the billet B abuts against the piercer plug 3, a hole is made in the central part of the billet B and the billet B is thereafter subjected to wall-thickness working between the piercer rolls 1a, 1b and the piercer plug 3, whereby a hollow shell S is obtained.
In such piercing-rolling, many faults occur at the start of rolling when the billet B is gripped by the piercer rolls 1a, 1b and rolling is started and when the rolling is finished and the rolled hollow shell S leaves the piercing-rolling mill 10. There are mainly the following two kinds of faults as faults occurring in piercing-rolling at the start of rolling.
In one fault, a fed billet B is not gripped by the piercer rolls 1a, 1b and does not abut against with the piercer plug 3 although the billet B comes into contact with the piercer rolls 1a, 1b. Hereinafter, this fault is called a slippage fault.
In another fault, the speed of the entry of the billet B into the piercer rolls 1a, 1b is slow or the entry stops although the billet B is gripped by the piercer rolls 1a, 1b and abuts against the piercer plug 3, and the rolling load of the piercer rolls 1a, 1b increases only gently after the billet B abuts against the piercer plug 3. Hereinafter, this fault is called a head jam fault.
Examples of manufacturing conditions for preventing the occurrence of such faults at the start of piercing-rolling include increasing the draft rate, which expresses the degree of gripping by the piercer rolls 1a, 1b. However, if the draft rate is made too high, an inner surface shell flaw (a flaw occurring on the inner surface of the shell) may occur.
The draft rate is defined as follows (see FIG. 2):Draft rate=(d−r)/d×100
where d is the outside diameter of the billet, and r is the gap between the piercer roll 1a and the piercer roll 1b at the place where the leading end of the billet abuts against the piercer plug 3.
Examples of manufacturing conditions for preventing the occurrence of faults at the start of piercing-rolling include increasing the coefficient of friction with the billet B by applying an antislipping agent to the surfaces of the piercer rolls 1a, 1b. However, if the application of the antislipping agent is continued, an outer surface shell flaw (a flaw occurring on the outer surface of the shell) may occur due to the roughness of the piercer roll surface, and operation troubles may occur due to, for example, the entry of the antislipping agent into the bearings of the driving device (not shown) which rotates the piercer rolls 1a, 1b. 
It is desirable to increase the opening of the piercer rolls 1a, 1b as a measure to be taken when a slippage fault has occurred, whereas as a measure to be taken when a head jam fault has occurred, it is desirable to reduce the opening of the piercer rolls 1a, 1b in the case of a billet B made of an ordinary steel and to apply an antislipping agent to the piercer rolls 1a, 1b in the case of a billet B made of a high-alloy steel.
However, for example, in the case where the billet B is made of a high-alloy steel containing not less than 2 mass % of Cr, the appropriate range of the draft rate is very narrow and, therefore, it is difficult to avoid faults in piercing-rolling. Also in the case where the billet B is made of a carbon steel, the rolling condition changes according to the condition of rough piercer roll surfaces and the like and, therefore, it is difficult to avoid faults in piercing-rolling.
If such faults in piercing-rolling occur, in the worst case piercing-rolling is stopped and all billets B present in the production line from the heating furnace to the piercing-rolling mill 10 must be taken out of the line, causing great damage. For this reason, in the case where a fault in piercing-rolling occurred, it is desirable to immediately detect the occurrence of the fault and to take measures.
However, the detection of these faults in piercing-rolling is visually carried out by skilled workers and is influenced by the skill of the workers, posing the problem that the accuracy of detection is low.
There are also known methods of detecting a fault in piercing-rolling which involve making a judgment that a slip has occurred between the piercer rolls and the billet and detecting a fault in piercing-rolling if during piercing-rolling the current value of motors driving the piercer rolls becomes lower than a prescribed threshold value (refer to Patent Literature 1, for example).
However, with this detection method, it is impossible to detect faults at the start of piercing-rolling as described above.